1. Field of the Invention
This invention relates generally to light beam deflection devices, and more particularly, is directed to an improved light beam deflection device employing an acoustooptic element for deflecting a light beam incident upon the acoustooptic element.
2. Description of the Prior Art
There have been proposed so-called optical writable discs of several types, each of which is available for writing an information signal therein and reading an information signal therefrom by means of a light beam incident thereupon. The optical writable disc of one type is provided thereon with a plurality of wobbling pits which are recorded by a light beam for recording caused to impinge upon the optical writable disc in a process of production of the same. The wobbling pits are used for a tracking servocontrol of a light beam impinging upon the optical writable disc for recording thereon or reading therefrom an information signal after the process of production.
For recording the wobbling pits on the optical writable disc, it is required to deflect the light beam for recording at a predetermined angle in synchronism with each wobbling pit. In the case where an acoustooptic deflector is used for deflecting the light beam for recording, the light beam for recording must be caused to enter the acoustooptic deflector at a glancing angle which is defined in relation to a lattice plane in the acoustooptic deflector and satisfies the Bragg condition expressed with the equation: 2.multidot.d.multidot.sin.theta.=n.lambda., where d represents a spacing of lattice planes, .lambda. represents a wavelength of the light beam for recording incident upon the acoustooptic deflector, n represents an integer. (Such a glancing angle .theta. is called the Bragg angle.) Accordingly, when the acoustooptic deflector is so positioned that the light beam for recording enters the acoustooptic deflector along a straight optical path, the light beam for recording emanates from the acoustooptic deflector along another straight optical path which extends at an angle of 2.multidot..theta. with the straight path along which the light beam for recording enters the acoustooptic deflector.
FIG. 1 shows a situation of light beam deflection caused by the acoustooptic deflector in such a manner as described above. In FIG. 1, an acoustooptic element 20 which forms the acoustooptic deflector comprises a piezoelectric crystal base 20A made of rock crystal, tellurium dioxide (TeO.sub.2) or the like and a transducer 20B attached to the piezoelectric crystal base 20A. The transducer 20B is connected to a signal input terminal 20C to which a signal Ss having a frequency of, for example, 200 to 300 MHz is to be supplied. When the signal Ss is supplied to the signal input terminal 20C, a diffraction grating 23 is substantially formed in the piezoelectic crystal base 20A by a supersonic wave arising in the same. Under a situation wherein the diffraction grating 23 is formed in the piezoelectic crystal base 20A, an incident light beam 21 directed to enter the piezoelectic crystal base 20A from an incident plane 20i thereof, which is perpendicular to each lattice plane of the diffraction grating 23, at an incident angle .theta. satisfying the Bragg condition, is deflected by the diffraction grating 23 formed in the piezoelectic crystal base 20A so as to emanate from an outgoing plane 20o of the piezoelectic crystal base 20A, which is also perpendicular to each lattice plane of the diffraction grating 23, as an outgoing light beam 22 directed to be at an angle of 2.multidot..theta. with the incident light beam 21.
As described above, in the case of the acoustooptic deflector composed of an acoustooptic element provided with a body member such as the piezoelectric crystal base 20A shown in FIG. 1, an incident light beam is required to be directed to enter the acoustooptic element from an incident plane thereof accurately at an incident angle satisfying the Bragg condition and, as a result of deflection, an outgoing light beam emanating from an outgoing plane of the acoustooptic element is directed to be at an angle corresponding to the double of the incident angle with the incident light beam. Consequently, it is very difficult to arrange properly and precisely the acoustooptic element and light beam paths of the incident and outgoing light beams.
Further, a transducer provided to the acoustooptic element, such as the transducer 20B shown in FIG. 1, is often restricted in size in view of designing so as to be almost equal in area to the cross section of the incident light beam at a diffraction grating in the acoustooptic element and therefore the outgoing light beam is subjected to astigmatism. This results in a disadvantage that the cross section of the outgoing light beam may be deformed to be elliptic, and in the case where the outgoing light beam having its cross section deformed to be elliptic is used for recording the wobbling pits on the optical writable disc, there is caused a problem that each of the wobbling pits cannot be provided with a proper shape.